1. Field of the Invention
The present invention relates to an impedance matching device for performing impedance matching between a high-frequency power supply and a load.
2. Description of the Related Art
A plasma processing system for forming a thin semiconductor film or etching is conventionally known. In such a plasma processing system, high-frequency power is supplied from a high-frequency power supply to a plasma processing apparatus to generate plasma in the plasma processing apparatus. To efficiently supply high-frequency power from the high-frequency power supply to the plasma processing apparatus during the plasma processing, an impedance matching device is provided between the high-frequency power supply and the plasma processing apparatus. The impedance matching device operates to match the impedance of the high-frequency power supply and that of the plasma processing apparatus.
In the plasma processing system, the peak-to-peak value Vpp (hereinafter referred to as “p-p value”) of the high-frequency voltage at the input port of the plasma processing apparatus can indicate the condition of the semiconductor manufacturing process. The input port of the plasma processing apparatus is directly connected to the output port of the impedance matching device. Thus, in a conventional plasma processing system, the output port of the impedance matching device is provided with a Vpp detector for detecting the p-p value Vpp of the high-frequency voltage vout outputted from the output port to the plasma processing apparatus.
For instance, JP-A-2008-300322 discloses an impedance matching device, which has the structure shown in FIG. 6.
The impedance matching device 100 shown in FIG. 6 incorporates an impedance matching circuit 101a comprising a T-shaped circuit. The impedance matching circuit 101a is made up of an inductor L and two variable capacitors C1, C2 which are connected into a “T” shape. Between the variable capacitor C2 and the output terminal RF-OUT are provided a Vpp detector 101b and a Vdc detector 101c. The Vdc detector 101c detects the DC bias Vdc of the high-frequency voltage vout. The Vpp detector 101b detects the p-p value Vpp of the high-frequency voltage vout at the output terminal RF-OUT. The p-p value Vpp is inputted into the controller 101d. The controller 101d controls the ignition in starting discharge and the impedance matching operation during the plasma processing in the plasma processing apparatus directly connected to the output terminal RF-OUT. Specifically, to assist the ignition in starting the discharge, the controller 101d adjusts the capacitances of the variable capacitors C1, C2 to increase the p-p value Vpp while monitoring the p-p value Vpp inputted from the Vpp detector 101b. 
In the structure disclosed in JP-A-2004-85446, the output port of the impedance matching device is provided with a high-frequency detector for detecting the high-frequency voltage, high-frequency current and phase difference between the high-frequency voltage and the high-frequency current. The detection results are used for detecting abnormalities such as an abnormal voltage or for controlling process parameters such as high-frequency power, gas flow rate, gas pressure supplied to the plasma processing apparatus. Since the high-frequency detector detects the instantaneous value of the high-frequency voltage, the high-frequency detector performs the function of detecting the p-p value Vpp of the high-frequency voltage.